"Administrator Pruitt’s testimony, viewed in the most charitable light, depicted a chief executive who has failed to exert any oversight over his staff as they have, as he testified, spent exorbitant funds and made impactful personnel decisions without his knowledge or approval,” the senators wrote. “His ready willingness to pass blame on to everyone around him, including career staff and his top political aides, shows that he is either unwilling or unable to lead.”

The senators noted that Pruitt has yet to testify before the panel on this year's or next year's budget, and they accused him of offering testimony that had been contradicted by earlier testimony and documents in his previous appearance.

"While we may disagree on the policy choices made by Administrator Pruitt, we hope we can agree that cabinet officials of any party owe Congress clear and truthful answers to questions and should be held to the highest ethical standards as Administrator Pruitt himself acknowledged last week," the senators wrote in their letter.

"We should also be able to agree that claims of accountability are meaningless unless they can be credibly backed up by meaningful oversight and investigations of misconduct."

Pruitt is slated to testify on Wednesday before the Senate Appropriations Subcommittee on Interior, Environment and Related Agencies.

Carper, the ranking member on the Environment and Public Works panel, told reporters Tuesday that he was disappointed that Pruitt would not also be coming before his committee in addition to the Appropriations subcommittee.

"My only disappointment is that it's not before the Environment and Public Works and my hope is that before long it will be because there is plenty to talk to him about," Carper said. "It's not just concerning to Democrats, its' also to a lot of Republicans. And maybe sooner rather than later they'll find their voices."

The EPA's inspector general sent a letter to Carper and Whitehouse on Monday in response to a request the two senators had previously sent about Pruitt's use of a 24/7 security team.

The inspector general said that Pruitt has been using the security detail since the day he started as EPA administrator. The letter appeared to contradict Pruitt’s testimony before the House that threats made against him during his time as administrator were the reason for the detail.